The scope of a pressure vessel applies to any container having pressure differential between its inside and outside. The contained energy is proportional to the pressure and volume of the fluid stored inside. The vessel requires proper structural reinforcement to balance the stored energy. While high strength materials provide the necessary strength at lower weight, an additional parameter, the energy of fracture of the total structure must be high enough to effectively control the failure. High toughness materials are therefore necessary for the construction of a pressure vessel to exceed the fracture mechanics based ‘leak-before-fail’ requirement. Metals usually provide a good combination of strength, modulus and fracture toughness. Reinforcement of high pressure hose and pressure vessels by metal wires has a history of over 100 years as an example a reference of wire appears in U.S. Pat. No. 1,281,557 of October 1918 by Goodall.
High strength fibers such as carbon or glass fibers are currently in use to reinforce a metal or polymer core pressure vessel as described in U.S. Pat. Nos. 7,412,956, 7,219,812 and 5,822,838. Composite Overwrapped Pressure Vessels (COPV) are made by wrapping carbon or glass fibers over a polymer or metal core tank. There are however two disadvantages with these ceramic fibers. First is the cost as a premium resin and special processing are required to maximize the mechanical properties of glass or carbon fiber reinforced composites. Secondly, the composite has almost no plastic ductility thus low fracture toughness which makes the structure highly sensitive to minor surface defects or damage.
Compared to above ceramic fibers, recent high strength metal wires provide excellent combination of strength and plastic ductility. Tire cord quality wires have been made in the strength range of 2000 to 4000 MPa. Unlike ceramic fibers, metal wires have good longitudinal and torsional plastic behaviors and thus can easily be processed to a cable and used more efficiently. Inherent elastic-plastic ductilities allow effective load sharing between individual filaments. High strength ductile metal wires therefore deliver improved fracture resistance thus providing better safety at reduced cost.
Most high strength wires are made of carbon steel with minor alloying elements. They are processed to provide strength around 3500 MPa. U.S. patents such as U.S. Pat. Nos. 5,779,829, 7,082,978, 6,247,514 and others claim strength of steel wires in the 4000 to 5000 MPa range. Commercial automobile tires have been built with 4000 MPa and higher strength steel wires. Currently 3500 MPa strength steels wires in the diameter range 0.15 mm to 0.38 mm are used in most pneumatic radial tires for use on passenger, light truck, medium truck and other applications. Construction geometry of the cable and proper coverage by a polymer/elastomer provide adequate protection against environmental degradation such as corrosion.
Successful experience of over a century of the use of steel wires in wire ropes, pressure hoses, tires, conveyor belts has provided a good confidence to explore its use in reinforcing pressure vessels. These vessels can be used for the storage of fluid such as hydrogen, compressed natural gas (CNG), oxygen, LPG and others for automobile, aerospace and other critical applications. The scope of the proposed invention is to design safer, light weight and economical pressure vessels by using metal wires.
The present invention describes methods of using high strength metal wires to reinforce a pressure vessel to optimize weight, toughness and cost. It additionally proposes methods for the assembly of the vessel and protection against environment.
National and International guidelines such as ISO 11439 has broadly categorized pressure vessels in four types. Type I is all metal while Type II, III and IV use fibers to reinforce-wrap a polymer or metal core tank. The present invention addresses Type II, III and IV and similar designs for other applications.